As schematically shown in FIG. 6, a conventional press brake comprises a upper apron 7 (alternatively referred to as a upper table or a fixed table) fixed on a left and right side frame 5L, 5R respectively mounted on a base 3, and a lower apron 9 (or referred to as a lower table or a movable table) provided so as to oppose the upper apron 7, the lower apron being vertically movable.
Hydraulic cylinders 11L, 11R are provided on the left and right sides respectively to move the above lower apron 9 vertically. At appropriate positions of the lower apron 9, crowning cylinders 13 are provided to compensate for a downward curve in the lower apron 9 generated at the time of bending a sheet-shaped work W.
The same Figure shows a case in which two crowning cylinders 13 are provided, however crowning cylinders may be provided in the center of the lower apron 9 or the upper apron 7. That is, the crowning cylinders may be provided in an arbitrary number, and may be provided on either the upper apron 7 or the lower apron 9.
An upper tool 15 is installed on the bottom of the upper apron 7 and a lower die 17 is installed on the top of the lower apron 9.
According to the above-described construction, hydraulic oil is supplied to the left and right hydraulic cylinders 11L, 11R. As a result, the lower apron 9 is raised so as to engage the upper tool and lower die 15, 17 appropriately, such that the work W is bent and processed by the upper tool and lower die 15, 17.
When the work W is bent as described above, the upper apron 7 is inclined to curve so as to protrude upward and the lower apron 9 is inclined to curve so as to protrude downward. As a result, the bending shape of the work W becomes so-called "ship-shaped" so that the center portion thereof tends to be loose.
Thus, to correct the curve of the lower apron 9, a pressurized oil is supplied to the crowning cylinders 13 and the curve is corrected in a trial and error manner to make the upper and lower aprons parallel to each other, which consumes a significant amount of time.
Moreover, the bending angle of the work W can be arbitrarily set by controlling the engagement positional relation between the upper tool and lower die 15, 17. To control the engagement positional relation between the upper tool and lower die 15, 17, the ascent stop position of the lower apron 9 can be arbitrarily set.
That is, as schematically shown in FIG. 7, hydraulic pumps P are connected to the left and right hydraulic cylinders 11L, 11R. With respect to the connection of the hydraulic pump P to the left and right hydraulic cylinders 11L, 11R, separate hydraulic pumps may be connected thereto separately or a single hydraulic pump may be connected thereto through branching.
A connecting oil path 19, connecting to the hydraulic pump P and the left and right hydraulic cylinders 11L, 11R, is connected to a branch line 23, which is connected to an upper limit valve 21.
In the above upper limit valve 21, a valve 21V is pressed by a spool 21S against the pressure of a spring SP to control the degree of opening between the valve 21V and the seat SE, so that hydraulic oil flowing from the branch line 23 to a tank T is controlled.
The above upper limit valves 21 are provided on both sides of the lower apron 9 to control the ascent stop positions of both sides of the lower apron 9. To press the spool 21S of this upper limit valve 21, levers 25 are provided swingably in the vertical direction on both frames of the press brake 1 and one end of the lever 25 is in contact with the top end of the spool 21S.
Then, corresponding to the left and right levers 25, vertical guides 27 are provided on the frames of the press brake, and nut members 29 are provided so as to move vertically freely along the guides 27. A screw pestle 31, provided in parallel to the guide 27, is engaged with this nut member 29. On the top end of this screw pestle 31, a cap 33 which contacts the other end of the lever 25 is provided.
To rotate the screw pestle 31, the frame is equipped with a servo motor SM having a rotary encoder E as a position detector. The rotary shaft 35 of this servo motor SM and the bottom end of the screw pestle 31 are spline-engaged with each other for the screw pestle to be vertically movable.
Then, a contacting member 37 capable of contacting each of the left and right nut members 29 from below is provided on each of the left and right sides of the lower apron 9.
Thus, if the lower apron 9 is raised by supplying a hydraulic oil to the left and right hydraulic cylinders 11L, 11R, the contacting members 37 provided on the left and right sides of the lower apron 9 make a contact with each of the nut members 29 on both sides, so that the left and right screw pestles 31 are raised.
If the left and right screw pestles 31 are raised, the caps 33 swing each of the levers 25, so that the levers 25 press the spools 21S in the left and right upper limit valves 21. If the spools 21S are pressed, an opening is produced between the valve 21V and the seat 21SE in the upper limit valve 21, so that a pressurized oil is discharged from the branch line 23 into the tank T.
As the ascent of the lower apron 9 progresses, the opening between the valve 21V and the seat 21SE is increased gradually. Then, when the force of raising the lower apron 9 by pressures applied to the left and right hydraulic cylinders 11L, 11R is balanced with a sum of the weight of the lower apron 9 and a load used for the bending processing of the work W, the ascent of the lower apron 9 is stopped.
When the ascent of the lower apron 9 is stopped as described above, if the nut member 29 is moved upward by controlling the servo motor SM appropriately, the nut members 29 separate from the contacting member 37, so that the lever 25 is rotated counterclockwise by the action of the spring S. As a result, the opening between the valve 21V and the seat 21SE in the upper limit valve 21 is squeezed by the action of the spring SP. Consequently, pressures in the left and right hydraulic cylinders 11L, 11R rise so that the lower apron 9 is raised further.
Then, if as described above, the contacting member 37 makes a contact with the nut member 29 so as to enlarge the opening of the upper limit valve 21, the lower apron 9 stops ascending as described previously.
Conversely, when the contacting member 37 is in contact with the nut member 29 such that the lower apron 9 stops ascending, if the servo motor SM is rotated inversely to the direction mentioned above to intend to bring down the nut member 29, the screw pestle 31 relatively rises because the nut member 29 is in a contact with the contacting member 37 such that it does not go down. Consequently, with respect to FIG. 7, the lever 25 is rotated clockwise.
If the lever 25 is rotated clockwise, the opening of the upper limit valve 21 is enlarged further and the pressures in the hydraulic cylinders 11L, 11R drop so that the lower apron 9 descends. If the lower apron 9 descends so that the lever 25 returns to its original position, the lower apron 9 stops descending.
That is, by adjusting the vertical position of the nut member 29, it is possible to stop the lower apron 9 at a desired ascent position. By controlling the engagement positional relation between the upper tool and lower die 15, 17, it is possible to bend the work W at a desired angle.
Meanwhile, the left and right hydraulic cylinders 11L, 11R can be controlled separately and the transverse inclination of the lower apron 9 can be controlled.
When the bending processing of the work W is conducted with the engagement positional relation between the upper tool and lower die 15, 17 controlled as described above, the engagement positional relation between the upper tool and lower die 15, 17 can be calculated theoretically according to the bending angle of the work W.
However, when the bending processing of the work W is conducted, a deformation (deflection) may occur in the left and right side frames 5L, 5R of the press brake, the upper and lower aprons 7, 9, the upper tool and lower die 15, 17 and the like. It is therefore necessary to correct for the deflection.
Preceding examples regarding correction of the deflection are found in, for example, Japanese Patent Application Laid-Open No. 57-100820 (hereinafter referred to as preceding example 1), Japanese Patent Publication Laid-Open No. 3-54013 (preceding example 2), and Japanese Patent Application Laid-Open No. 6-26226 (preceding example 3).
The above preceding example 1 has disclosed an apparatus in which the amount of deflections of the side frames at the time of actual bending processing is detected to calculate an applied pressure and an appropriate amount of correction is calculated to control the engagement positional relation between the upper tool and lower die.
Further, the preceding example 2 has disclosed an apparatus for setting a deformation correction coefficient, based on an assumption that the amount of deformation of the side frame or the upper tool and lower die is parallel to the magnitude of pressure applied to the work, and controlling the engagement positional relation between the upper tool and lower die by using the deformation correction coefficient.
As in the preceding example 2, assuming that the amount of deformation is simply in a parallel relation to an applied pressure, a deformation correction coefficient must be obtained corresponding to the type of the upper tool and lower die or a combination thereof each time, so that there is a problem in improvement of the operation efficiency. Further, if the bottom end of the upper tool bites into a work (see lines 4-5 of the left column on page 6 of the preceding example 1), even if the deflection is corrected with an assumption that the amount of deformation is simply in a parallel relation to an applied pressure, there is a problem in improvement of bending accuracy.
According to the preceding example 3, by following a correction pattern based on a reference curve indicating a relation between the bending angle of the work and a D value for controlling the engagement positional relation between the upper tool and lower die, the engagement positional relation between the upper tool and lower die is controlled when the bending processing of the work is conducted. After the bending processing, an actual bending angle is measured, and based on this measurement result, the correction pattern is shifted to a new correction pattern following the reference curve.
As regards the above correction pattern, biting of the tip of the upper tool into the work is considered. Further, by measuring actual bending angles of the work, a new correction pattern is produced and then, the engagement positional relation between the upper tool and lower die is corrected based on this new correction pattern. As a result, the accuracy in bending is improved, thus this technique is preferable.
However, the correction pattern based on the above reference curve is a fixed pattern, and if any error is contained in this correction pattern, a new correction pattern obtained by the above shift also contains an error, which is not preferable in term of improvement of the accuracy in bending.
Further, a conventional bending processing apparatus equipped with a control unit for calculating and controlling the D value of a distance from the bottom of the die to the tip of the punch is well known as disclosed in Japanese Patent Application No. 1-20927. This invention is constructed according to the following experimental formula based on a limited database to obtain a final D value. EQU D(A)=D.sub.1 +(.delta..sub.1 +.delta..sub.2 +.delta..sub.3 +.delta..sub.4 +.delta..sub.5)
In the above formula, D.sub.1 is a geometrical D value. For general purpose, various factors are included, for example, .delta..sub.1, .delta..sub.3 are mechanical system deflection, .delta..sub.2, .delta..sub.4 are material property and .delta..sub.5 are other factor.
By the way, the aforementioned conventional control system lacks general purpose functionally because it depends on a limited database and sometimes does not correspond to diversified conditions. For example, if an experimental formula derived from this database is utilized for work thickness and materials other than those obtained from actual experiment, no satisfactory precision can be obtained.
Particularly, this database is incomplete because it contains only the tensile strength parameter as a means for expressing material property. Further, to secure a desired bending accuracy, at each time of correction, the parameter is modified. Thus, the frequency of trial bending increases so that necessary tact time is prolonged.